apm821xx: fix bogus key-presses on boot

apm821xx: fix bogus key-presses on boot

"There are often transient line events when
the system is powered up and initialized and it
is often necessary for the gpio_chip driver to
clear any interrupt flags in hardware before
setting up the gpio chip, especially the
irqchip portions of it."
<http://lists.infradead.org/pipermail/openwrt-devel/2019-June/017630.html>

This patch adds a fix for the APM821XX's interrupt
controller to clear any bogus pending toggled
interrupts that happens on various APM821XX boards
on boot.

The patch also changes the debouce-interval from the
default 5ms debounce interval to 60ms all around.
The default setting caused on occasions that the button
state became stuck in a pressed state, even though the
button was released.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
(backported from commit 971f31bedcad0a954a4a4e79140346cc896d352e)